(b) how the cell cycle is regulated

The idea

The cell cycle is an ordered sequence of interphase (G1, S, G2) and mitotic phases, and its progress is controlled at checkpoints so that DNA is replicated and segregated only when conditions are suitable. Regulation involves cyclins, whose concentrations rise and fall through the cycle, binding to and activating cyclin-dependent kinases (CDKs); the active complexes phosphorylate target proteins that move the cycle forward. Checkpoints, such as the G1 checkpoint assessing cell size, DNA damage and nutrient availability, the G2 checkpoint confirming replication is complete and undamaged, and the spindle assembly checkpoint during mitosis, can halt progress. Loss of control, for example through mutated proto-oncogenes or tumour suppressor genes, can lead to uncontrolled division and tumour formation.

What earns the marks

• Interphase consists of G1 (growth and synthesis of organelles), S (DNA replication) and G2 (further growth and checking), followed by mitosis and cytokinesis.
• Cyclins are proteins whose concentration varies cyclically; they bind to and activate cyclin-dependent kinases (CDKs).
• Activated cyclin–CDK complexes phosphorylate target proteins, driving progression through the cycle.
• Checkpoints (for example G1, G2 and the spindle assembly/metaphase checkpoint) monitor conditions such as DNA damage, completion of replication and chromosome attachment to the spindle, and can arrest the cycle.
• Failure of regulation, such as mutation of proto-oncogenes or tumour suppressor genes, can cause uncontrolled cell division and tumour formation.

Common mistakes and how to put them right

• Thinking CDKs fluctuate in concentration while cyclins stay constant.

  Put it right: In fact cyclin concentration varies and CDKs are present throughout, becoming active when bound to cyclins.

• Confusing the S phase with mitosis.

  Put it right: DNA replication occurs in S phase of interphase, not during mitosis.

• Assuming checkpoints only detect DNA damage.

  Put it right: They also assess cell size, nutrient availability and correct spindle attachment.

• Believing regulation is a single event.

  Put it right: It is continuous control at multiple checkpoints across the cycle.

Exam tips

• Learn the order G1 → S → G2 → mitosis → cytokinesis and state what happens in each phase.
• When asked about regulation, name cyclins, CDKs and checkpoints explicitly rather than describing the cycle generally.
• Link loss of regulation to named gene types (proto-oncogenes, tumour suppressor genes) to gain credit for cause and effect.